What causes stains on pool surfaces?
Pool stains fall into two groups. Metal stains — rust, copper, and iron, often from well water or corroding equipment — are brown, green, or grey and do not brush off. Organic stains from leaves, algae, and debris are usually brown or reddish and respond to chlorine and brushing.
Telling them apart
| Clue | Metal stain | Organic stain |
|---|---|---|
| Response to chlorine | No change or darkens | Lightens or lifts |
| Response to vitamin C tablet | Fades under the tablet | No change |
| Typical colour | Rust brown, grey, blue-green | Brown, red, purple |
| Common source | Well water, corroding rails or heater | Leaves, acorns, dead algae |
The well water problem
Topping a pool off with untreated well water introduces iron and other metals with every fill. They stay in solution until chlorine oxidises them, then they drop onto the plaster as stains. Using a hose pre-filter and a metal sequestrant is how you stop the cycle.
What removes them
- Organic stains: shock, brush, and keep chlorine at target for a few days
- Metal stains: ascorbic acid treatment plus a sequestrant to hold metals in solution
- Old, set-in stains on plaster: acid wash with the pool drained
- Prevention: keep pH in range and filter or treat any well water you add